Superconducting air-core, synchronous electric machines have been under development since the early 1960's. The use of superconducting windings in these machines has resulted in a significant increase in the field electromotive forces generated by the windings and increased flux and power densities of the machines.
Early superconducting machines included field windings wound with low temperature superconductor (LTS) materials, such as NbZr or NbTi and later with Nb3Sn. The field windings were cooled with liquid helium from a stationary liquefier. The liquid helium was transferred into the rotor of the machine and then vaporized to use both the latent and sensible heat of the fluid to cool the windings. This approach proved to be viable for only very large synchronous machines. With the advent of high temperature superconductor (HTS) materials in the 1980's, the cooling requirements of these machines were greatly reduced and smaller superconducting machines were realizable.
In addition to the heat generated by the rotor assembly, the stator assembly also generates a considerable amount of heat that must be removed in order for the superconducting machine to operate efficiently. In conventional “non-superconducting” rotating machines, iron teeth are utilized between the individual stator coil assemblies, which act as heat sinks and remove the heat generated by the stator assembly. However, in superconducting machines, the flux density is so great between these stator coil assemblies that these iron teeth would immediately become saturated, resulting in Eddy current heating and operating inefficiency.